Re: [RFC/PATCH] cpuset: cpuset irq affinities

Paul Jackson
2008-03-04T06:52:04Z

Paul M wrote:
> I'm one such user who's been forced to add the mem_hardwall flag to
> get around the fact that exclusive and hardwall are controlled by the
> same flag. I keep meaning to send it in as a patch but haven't yet got
> round to it.

I made essentially the same mistake twice in the evolution of cpusets:
1) overloading the cpu_exclusive flag to define sched domains, and
2) overloading the mem_exclusive flag to define memory hardwalls.

I eventually reversed (1), with a deliberately incompatible change
(and you know how I resist those ;), creating a new 'sched_load_balance'
flag that controls the sched_domain partitioning, and removing any
affect that the cpu_exclusive flag has on this.

Perhaps the unfortunate interaction of mem_exclusive and hardwall is
destined to go the same path. Thought the audience that is currently
using mem_exclusive for the purpose of hardwall enforcement of kernel
allocations might be broader than the specialized real-time audience
that was using cpu_exclusive for dynamic sched domain isolation, and so
we might not choose to just break compatibility in one shot, but rather
phase in your new flag, before, perhaps, in a later release, phasing
out the old hardwall overloading of the mem_exclusive flag.

(My primeval mistake was including the cpu_exclusive and mem_exclusive
flags in the original cpuset design; those two flags have given me
nothing but temptation to commit further design errors ;).

> Also, if you're using fake numa for memory isolation (which we're
> experimenting with) then the correlation between cpu placement and
> memory placement is much much weaker, or non-existent.

That might be a good answer to my asking where the beef was.

--
I won't rest till it's the best ...
Programmer, Linux Scalability
Paul Jackson <pj@sgi.com> 1.940.382.4214